1. Field of the Invention
The present invention relates to an image pick-up apparatus such as a digital camera or a surveillance camera having an autofocus (AF) function, and to a control method for the image pick-up apparatus.
2. Description of the Related Art
A focus detection device in early stages of an image pick-up apparatus, such as a camera, was provided with one focusing point at the center of a taking area. The number of focusing points increased gradually according to development of a focus detection device, it progressed to three points, seven points, nine points, nineteen points, and reached forty-five points, which has broadened a possible area for focus detection within the taking area.
Such an increase of the focusing points has the following advantages. That is, it can avoid so-called “out-of-focus in center” situation in which a camera focuses on a background and not on a subject in normal shooting. Further, it decreases a frequency of losing a subject from the focusing points when shooting a moving subject.
A multi-point AF (autofocus) system using a phase difference focuses on the subject located at the nearest position among a plurality of subjects that are coincident with the focusing points, in general. Accordingly, for example in person photography, when another body such as a flower or a ball is located at the nearest position, there is a problem that the camera focuses on the other body and not on the person.
Therefore, in recent years, AF systems using an eye control method and a subject (a face etc.) detection technique have been proposed, and such a system enables to take a picture that is well focused on a target.
For example, there is a technique to extract a face image from an inputted image using a face detection method of automatically detecting a face region by detecting characteristic parts of a face or skin color parts of a subject person in taken image data. Japanese laid-open patent publication (Kokai) No. 2004-13871 (JP2004-13871A) discloses a technique to register specific image data as an reference image, and to determine whether newly inputted image data agrees with the reference image.
Japanese laid-open patent publication (Kokai) No. 9-251534 (JP9-251534A) discloses a technique of specifying a region including a face by calculating a correlation value between face image data that has been registered beforehand and image data of a discrimination target. Japanese laid-open patent publication (Kokai) No. 10-162118 (JP10-162118A) discloses a technique of dividing image data into a plurality of regions by a frequency band, limiting regions that have high possibilities of existence of a face, and then calculating a correlation value between face image data that has been registered beforehand and image data in each of the limited regions.
Japanese laid-open patent publication (Kokai) No. 2003-107335 (JP2003-107335A) discloses an example in which these techniques are applied to a camera. In the example, a main subject is automatically detected from image data using a technique such as a shape analysis. Then, the camera indicates a focusing point on a monitor display so as to overlap the detected main subject, and executes a focus adjustment process on the indicated focusing point.
Japanese laid-open patent publication (Kokai) No. 2004-317699 (JP2004-317699A) discloses a technique to discriminate a region in which a face as a main subject exists by extracting characteristic points from image data, and to set a focus detection area according to the size of the discriminated region. Japanese laid-open patent publication (Kokai) No. 2004-320286 (JP2004-320286A) discloses a technique to discriminate a region in which a face as a main subject exists by extracting characteristic points from image data, and to drive a zoom mechanism according to the size of the discriminated region.
Japanese patent publication No. 3097250 (JP3097250B) discloses a technique to track a subject by a photometric/colorimetric device and to display a tracking result on a display device whose display area is divided into fifty-four regions. Since this display device is arranged between a focusing screen and a pentagonal prism, a user can observe a subject image on which the tracking result is superimposed, through an optical finder.
A compact-type digital camera, which does not have an optical finder and whose image sensor also acts as a focusing sensor, displays a detection result of a subject on a liquid crystal display monitor mounted on the camera back, and can adjust a focus by selecting a focusing point automatically or intentionally.
On the other hand, a single lens reflex digital camera is provided with an optical finder and a focusing sensor for the AF operation in addition to the image sensor. The focusing sensor is used to perform the AF operation when a main mirror, which is arranged inside the camera, is located in a taking optical path to guide a light to the optical finder. The image sensor is used to perform the AF operation when the main mirror goes out from the taking optical path.
In this case, if the face detection is tried to be performed using the image sensor, the image outputted from the image sensor is displayed on a liquid crystal display monitor in a so-called live view shooting mode, and then, the AF operation is performed using the image sensor on the face region detected from the image. Therefore, such an AF operation needs a longer time until the camera focuses on a subject as compared with the case where the AF operation is performed using the focusing sensor, which causes a problem of increasing a release time lag.
In the above-mentioned Japanese patent publication No. 3097250 (JP3097250B), the display area is divided into a plurality of regions, and an index that indicates a detection of the subject in a region near the detected subject is displayed. However, since only one index (frame) is illuminated in this technique, there is a problem in which it is unclear whether the subject is caught as a region.